When using photodiodes in a receiver system using a carrier such as FM it is often desirable to tune the photodiode capacitance at the frequency of interest with an inductor in parallel. With such an arrangement Q's of 30 to 50 are easily realizable. A Q of 30 would correspond to a 3 bD bandwidth of 3.3 percent of the operating frequency and a gain in signal voltage of 30 dB over an untuned photodiode. In portable equipment using such high Q's, the resonant frequency of the input circuit will vary more than generally be tolerated due to changes in battery voltage and temperature affecting the capacitance of the photodiode. In addition, it would be desirable at the same time to improve the bandwidth and response of the receiver over that of a high Q single tuned circuit.
Therefore, there is a need to tune the resonant frequency of the input circuit wherein the effect of temperature and supply voltage are minimized while improving on the passband response of a single high Q tuned circuit.